The present invention is directed, in general, to well logging techniques using injected alternating current to produce time varying magnetic fields having characteristics which correspond to anomalies within the earth, and to apparatus for determining the location of such anomalies with respect to a well being drilled. More particularly, the invention is directed to flux gate magnetometers for detecting the total magnetic field in the earth, to improved systems and circuitry for producing electric signals corresponding thereto, and to circuitry for analyzing the signals to determine the distance and direction from a well being drilled to an anomaly.
The detection of magnetic fields within boreholes or wells for the purpose of analyzing surrounding earth formations or nearby bodies within the earth has a long history; a large number of patents have issued and numerous articles have been published on various devices and techniques for carrying out such analysis. Many of the known procedures have been concerned with the measurement of essentially static; i.e., time independent, magnetic fields such as the earth's magnetic field or the fields which are produced by the magnetic characteristics of various anomalies that occur in earth's formations. In addition, numerous patents and publications have been directed to the measurement of time varying magnetic fields produced by impressed alternating currents flowing in the ground. For example, U.S. Pat. Nos. 1,902,265 and 2,261,563, both to Rieber, disclose the use of low frequency alternating currents injected into the earth to detect anomalies in the earth's conductivity and to locate geological formations having a conductivity different from that of the surrounding formation. The electrical current is passed through the earth, and the magnetic fields produced by this current are mapped by means of a magnetometer at the earth's surface. In both patents, electrodes are used to inject the current, and in the '563 patent, one of the electrodes is a drill which penetrates deeply into the earth. Measurement of the signals is obtained by a manual nulling technique.
U.S. Pat. No. 1,803,405 to Ricker illustrates that electromagnetic fields produced by alternating currents have been used to locate masses, such as metallic ore bodies, beneath the earth's surface wherein such masses have electrical constants which were different from the constants of the surrounding earth. The magnetic field produced by an alternating current flow through the earth is detected, in the Ricker patent, by means of an induction coil which is mounted for rotation on the surface of the earth so that measurements can be made in different directions. A magnetic compass is affixed to the coil for orientation purposes. Similar induction coil detectors for measuring varying magnetic fields within the earth are illustrated in U.S. Pat. No. 2,359,894 to Brown et al and U.S. Pat. No. 2,291,692 to Cloud. In both of these patents, the magnetic field to be measured is produced by an alternating current injected into the earth either by a coil or by electrodes, but the induction coil detectors are located in boreholes, rather than on the surface. In Brown et al, the outputs of the induction coils are amplified downhole, and the signals are supplied to a receiver at the surface, while in Cloud the coil outputs are amplified at the surface.
U.S. Pat. No. 2,273,374 to Williams discloses apparatus for electromagnetic exploration of the earth from a borehole wherein two electrodes are positioned in a vertical borehole at spaced locations. A low frequency alternating current voltage is passed through the earth in the vicinity of the borehole from these electrodes and a pair of magnetic field sensors arranged at right angles to each other and to the axis of the device are positioned in the hole below the electrodes to detect alternating magnetic fields produced by the injected current. Measurement of the amplitude and phase of the voltages obtained from the two sensors permits determination of the amplitude, phase, and relative direction with respect to the sensors of the alternating horizontal magnetic field vector resulting from a conductive anomaly in the earth. In order to determine the azimuthal direction of the anomaly, the compass direction of the axes of the sensors must be known, and for this purpose an orientation device such as a compass is also provided by Williams.
U.S. Pat. No. 3,369,174 to Groenendyke et al discloses a logging system wherein a time-varying magnetic field is produced in the earth surrounding a borehole, and a magnetometer is used to measure the total magnetic field encountered in the borehole as the magnetometer is moved along the length of the hole. The output of the magnetometer includes components produced by the earth's magnetic field, by the artificially created time varying magnetic field, and by modulations of those fields caused by the motion of the logging tool. These output signals are fed to surface instrumentation which separates the signals representing the artificially created time-varying field from the signals representing the earth's field and varying formation fields. The time-varying field signal is a composite signal having components which are separated to obtain the desired information concerning the formations being measured.
A further teaching of the use of fluxgate magnetometers for detecting an artificially created magnetic field within the earth is illustrated in U.S. Pat. No. 3,406,766 to Henderson. This patent discloses the use of two fluxgates positioned at right angles to each other, with the downhole package including suitable electronics for actuating the fluxgates and for amplifying the signals received therefrom. The orientation of the two fluxgates allows measurement of X and Y components of the total magnetic field to provide a measure of the distance and the direction to a target. U.S. Pat. No. 3,731,752 and U.S. Pat. No. 3,701,007, both to Schad, are similar to Henderson in showing the use of magnetometers for measuring the total magnetic field in a borehole for the purpose of locating a target source. In the Schad patents, downhole electronics are provided to drive the magnetometers and to separate the signals representing the earth's magnetic field from signals representing a superimposed alternating magnetic field, and surface instrumentation provides a measure of these fields. In both the Henderson and Schad patents, the target to be located is a magnetic field having its source in a second borehole.
U.S. Pat. No. 3,745,446 to Norris discloses a logging technique for locating the tops of buried well casings. This device utilizes a pair of vertically spaced magnetometers which are mounted in a test borehole for rotary motion so as to sweep the magnetometers through a full circle to detect the static magnetic fields surrounding the test hold. By summing the output signals of the spaced magnetometers and plotting the result, the gradient of the magnetic field having as its source a buried casing is obtained, and this value is plotted to obtain an indication of the depth of that casing as well as its distance and direction from the test borehole.
U.S. Pat. No. 3,510,757 to Huston discloses an induction coil array for determining the X and Y components of the magnetic fields present in the region of the borehole. Variations in the conductivity of earth formations due to anomalies in the formations cause variations in the flow of current produced by a down-hole transmitting coil driven by an AC input, and thus in the secondary magnetic fields generated thereby, and these magnetic fields are detected by the induction coil array. A compass, which may be a fluxgate device, provides information concerning the orientation of the receiving coils so that the direction to the source of a magnetic field produced by an anomaly can be determined. The outputs from the induction coils are amplified in a downhole electronics package, the signals are detected in a phase sensitive detector, and the detected signals are sent uphole for further processing.
U.S. Pat. No. 4,072,200 to Morris et al discloses a survey system for locating target subterranean bodies exhibiting a static magnetic field, a time varying magnetic field, or an electric field. The device includes four sensors, the first two being perpendicular to the axis of the survey tool for measuring X and Y components of static magnetic fields produced, in part, by the remanent magnetism of a target subterranean body, and the other two being spaced apart and aligned with each other along the longitudinal axis of the tool for measuring the gradient of the static field along a borehole. In addition, a single AC sensing coil is carried by the tool to detect and measure alternating magnetic fields produced at a target body. In the Ac mode of operation, the direction to the target is determined by selecting the orientation of the instrument so that a minimum response is detected by the AC sensing coil. The output from the AC sensor is amplified and transmitted to surface instrumentation, where the AC signals are converted to DC and fed to a calculator. Output signals representing the static fields are obtained from the static field sensors via suitable detector and amplifier circuitry, and are also transmitted to the surface calculator.
U.S. Pat. No. 3,791,043 to Russell discloses an instrument for measuring the inclination and direction of a borehole. The device uses fluxgate sensors for measuring the components of the earth's magnetic field, and in the preferred form of the invention, three mutually perpendicular fluxgate magnetic sensors are used. When the instrument is used in deep boreholes, the measured data are transmitted to the surface by means of pulse code transmission
U.S. Pat. Nos. 4,323,848, 4,372,398, 4,443,762, 4,529,939 and 4,700,142, all of which issued to Arthur F. Kuckes, are directed to apparatus and techniques for measuring time varying magnetic fields in boreholes, and more particularly to the use of magnetometers of measuring the X and Y components of such fields. In U.S. Pat. No. 4,323,848, X and Y components of a time varying magnetic field and X and Y components of the earth's magnetic field are measured, the time varying field being generated by current flow in the earth produced by an AC source at the earth's surface. The outputs from induction coil time varying magnetic field sensors and from static magnetic field sensors are supplied sequentially to a voltage controlled oscillator for transmission to the surface.
The separate measurements of a time varying magnetic field and of the earth's magnetic field described in the '848 patent and in other patents disclosing such field measuring systems as discussed above result in large differences in output signal levels from the respective sensors, for the earth's magnetic field normally is very large compared to AC magnetic fields being detected. Separate amplifiers and filters are used in such systems to transmit these output signals to the surface, but such systems allow drift, such as might be due to temperature changes within the borehole, aging of components, and the like, to affect the signals differently over a period of time so that inaccuracies can occur.
In U.S. Pat. No. 4,372, 398, fluxgate magnetometers are used as magnetic field sensors, and current injecting electrodes are provided in the borehole to produce a time varying magnetic field at a target anomaly. In one embodiment, four magnetometers produce four analog output signals, two representing the X and Y axes of the total sensed magnetic field, respectively, and two outputs representing the X and Y axes of the earth's magnetic field, respectively. The sum of the X axis outputs, which represents the composite X axis field component, and the sum of the Y axis outputs which represents the composite Y axis field component, drive corresponding X and Y axis voltage controlled oscillators which transmit the respective X and Y composite signals to the surface for processing. Surface instrumentation demodulates the X and Y signals and analyzes the waves to determine the distance and direction to the source of the time varying magnetic field at the target anomaly. In this system, the X axis and the Y axis signals are transmitted to the surface on separate wires, so that two signal transmission lines are required. Furthermore, the disparity between the extremely low amplitude of the alternating current field and the relatively high amplitude of the earth's magnetic field makes it difficult to reliably track the alternating current field, thus effectively reducing the sensitivity of the device. The use of voltage controlled oscillators also can reduce the accuracy of the device because such oscillators are sensitive to high temperatures.
Although the art of deep well magnetic logging is highly developed, as indicated above, the many devices described above operate with varying degrees of success. Those devices which do not provide electronic circuitry in the borehole to filter and boost the output signals from the magnetic field sensors are limited in the depth at which they can be used effectively, for any need to send very small signals along the downhole cables without amplification severely limits the available sensitivity of the devices. On the other hand, it has been found that the environmental conditions in deep boreholes are detrimental to electronic circuits, and the use of amplifiers, oscillators, and the like is severely compromised by temperature variations and other ambient conditions so that the processing and transmission of analog signals can introduce numerous uncertainties and errors.
The high sensitivity and large dynamic range of fluxgate magnetometers provides a significant advantage to the use of such sensors, particularly in applications where subterranean targets are to be located at long distances and under adverse environmental conditions. However, before these characteristics can be used to advantage, there is a need to provide a significant increase in the sensitivity, accuracy and reliability of the circuitry used for handling the signals produced by such devices. There is a particular problem in borehole operations such as drilling of relief wells for the location of blowouts, for the depths at which the magnetic field sensors are used can be in excess of 15,000 feet and the adverse conditions in such wells severely degrade the operation of the electronic circuits and this adversely affects the measurements being made. Thus, these conditions often cause circuit noise and drift which mask low level signals, limiting the range of the sensor device and reducing its accuracy. Furthermore, where an impressed time varying magnetic field is to be detected at large distances, the desired field is usually very small in comparison to the earth's magnetic field, and because of this disparity, the measuring circuitry often cannot provide accurate and reliable measurements of both the time varying magnetic field and the static magnetic field.